Next generation sensor networks may be powered by energy harvesting techniques to avoid requiring battery maintenance. Energy harvesting is a process by which energy is derived from external sources (e.g., radio frequency energy, solar power, thermal energy, wind energy, salinity gradients, or kinetic energy), captured and stored.
Energy may be harvested from radio frequency signals propagating wirelessly. With RF harvesting, wireless energy comes from a radio frequency transmitting device that is some distance away from a device that harvests energy from the radio frequency transmission. Properties of an energy harvester include its ability to harvest energy efficiently from available RF signals, its ability to store the harvested energy efficiently with minimal storage loss, and its ability to make the stored energy available to meet the voltage, current, and duty cycle requirements of a desired application.
One of the more popular forms of RF used today is Wi-Fi (also referred to as IEEE 802.11a/b/g/n etc.) communications. Today, most Wi-Fi communications are in the 2.4 GHz and 5.8 GHz frequency bands and there are many local area networks that are based on Wi-Fi in which access points enable Wi-Fi clients to gain access to networks such as the Internet. Furthermore, the 2.4 GHz and 5.8 GHz bands also support other networking standards, such as Zigbee and Bluetooth, and other proprietary networks, each transmitting energy by communicating in this same frequency band. Additionally there are other frequency bands that support different communication protocols, each of which transmit energy when they are communicating. These include, for example, digital television (DTV) and Global System for Mobile Communications (GSM) signals.
In a traditional RFID-like setup the RFID readers (interrogators) usually deliver RF power as a continuous wave (CW), i.e. a sinusoidal signal of a particular frequency. In contrast to systems designed for use with continuous wave (CW) signals such as RFID, some RF energy harvesters are designed for use with non-CW signals such as WiFi and GSM. These waveform aware harvesters include ambient energy harvesters that harvest signals of opportunity that are already present in the environment. For these types of harvesters, the system designer must work around the waveforms and power levels that are already present in the environment.